ww test: allow -c with persistent workdirs

docs/spec.md

@@ -665,6 +665,9 @@ destinations and removes temporary stages and invocation-created output
 prefixes. Test execution starts only after that transaction commits, so a
 runtime failure leaves an explicitly retained binary. A no-test product
 publishes no binary and does not create a directory solely for one.
+`-w` may persist the unchanged semantic actions for either `-c` or running
+retention without changing publication identity or introducing a test-result
+cache.
 
 When `ww test` executes a directory-owned product, the child process working
 directory is that product's canonical absolute physical package source